In variable displacement refrigerant compressors, displacement or capacity control is provided by controlling the refrigerant gas pressure differential between the backside of the pistons or crankcase and compressor suction. A suction pressure biased control valve is most commonly used to control this pressure differential. In such control valves, the suction pressure operates on a diaphragm or evacuated bellows so that when suction pressure increases (usually associated with higher ambient temperatures and/or lower rotating speeds) it causes the control valve to effect decreased crankcase-suction pressure differential by allowing the crankcase to bleed gas to suction. As is understood by those in the art, the gas bleeding to suction has the effect of increasing the wobble plate angle and, thus, compressor displacement. Eventually, maximum compressor displacement is obtained when there is effected zero crankcase--suction pressure differential. On the other hand, when the air conditioning capacity demand is lowered (usually associated with lower ambient temperatures) so is the suction pressure. The control valve operates under the influence of the lowered suction pressure to close off the crankcase gas bleeding to suction so as to effect an increased crankcase--suction pressure differential. An increase in the crankcase--suction pressure differential has the effect of reducing the wobble plate angle and thereby decreasing compressor displacement.
To be accepted by Industry, a variable displacement automotive-type air conditioning compressor must be capable, during initial pull down, of approaching the same low evaporator pressures and temperatures as those obtainable by fixed displacement compressors. With a variable displacement compressor, a low suction pressure control point may be necessary and feasible during high ambient temperature operation, but it cannot safely be maintained into the lower ambient temperature range. In the high ambient temperature range, the heat load on the evaporator and the mass flow rate through the cooling system are high. A high pressure drop in the suction line is also experienced when the compressor is operated in high ambient temperatures. The high pressure drop in the suction line keeps evaporator pressures and temperatures up and out of the freeze up region. Of course, as ambient temperature decreases, so does the mass flow rate in the cooling system and the pressure drop in the suction line. As evaporator pressure converges on the low suction control point, the probability of evaporator freeze up increases. Accordingly, there is a need for a valve arrangement which controls compressor displacement in a variable displacement compressor so as to maintain a near constant evaporator pressure regardless of ambient temperature.
Various control valves for regulating the flow of fluid from the crankcase to suction in a wobble plate compressor are taught in the art. Dual acting valves responsive to the suction discharge pressure differential are also known in the art.
U.S. Pat. No. 3,959,983 discloses communication between crankcase and suction cavities to provide wobble plate control. In the U.S. Pat. No. 3,959,983 a pump operates to the relief limit of a valve and pumps oil to a chamber with a stroke control piston assembly. A zero--stroke valve is actuated by oil pressure and controls the gas flow from the crankcase to a control valve in response to oil pressure. The control valve is responsive to the pressure sensed in the suction line by a diaphragm pressure disposed to move a valve element. Fluid at discharge pressure blows by and is communicated past the pistons to the crankcase from the discharge cavity.
U.S. Pat. No. 4,037,993 teaches the communication of discharge gas pressure through a centered passageway to move a piston against a biased spring. A control valve assembly controls the flow of discharge or suction gas pressure to the center passageway in response to crankcase pressure. The control valve assembly includes a bellows valve which further provides communication between the suction cavity and the crankcase cavity.
U.S. Pat. No. 4,073,603 illustrates a control valve assembly utilizing a solenoid operator and a Sylphon bellows. This control valve regulates fluid pressure between the crankcase and the suction port.
U.S. Pat. No. 4,145,163 discloses a control valve for a swash plate compressor, which swash plate translates on a drive shaft. A bellows operated slide valve communicates fluid at discharge pressure to the crankcase in response to changes in the fluid suction pressure.
U.S. Pat. No. 4,428,718 teaches a control valve for a variable capacity wobble plate compressor. The valve arrangement in this patent includes a three chamber arrangement. The fluid discharge and fluid suction pressures are biased against the crankcase pressure in an intermediate chamber, as well as each other, to provide a control means for the wobble plate. This device utilizes a bellows valve in the suction pressure chamber and a spring-loaded ball valve in the discharge pressure chamber with a mechanical connection therebetween to bias each other.